Insect pathogenic nematodes of the families Heterorhabitidae and Steinernematidae are known to be symbiotically associated with bacteria of the genus Xenorhabdus, and it has been observed that these bacteria have the ability to inhibit the activity of other bacterial genera. Xenorhabdus are described by Thomas and Poinar, Int J. Syst. Bacteriol., 29 (4), 352-360, (1979),Akhurst, Int. J. Syst. Bacteriol., 33 (1), 38-45, (1983), Akhurst, J. Gen. Microbiol., 121, 303-309, (1980), and Akhurst, J. Gen. Microbiol., 128, 3061-3065, (1982). The activity is believed to derive from antibiotic substances released by the Xenorhabdi. Paul et al. (J. Chem. Ecol, 7 (3), 589-594 (1981), for example, have confirmed the presence of certain indoles and stilbenes in cultures of X. nematophilus and X. luminescens, and have shown that these compounds are active against a number of non-pathogenic, bioluminescent bacteria.
Paul et al. suggest that several antibacterial mechanisms may be operating in the Xenorhabdus systems. This possibility could explain the fact that we have succeeded in isolating from X. nematophilus, antibiotic compounds of quite different structure from those investigated by Paul et al. The new compounds have, moreover, proven to be active against a wide range of bacteria, including Gram positive species and are also effective as pesticides, especially as insecticides.
Bacteria of the genus Xenorhabdus are found to occur in two forms, called primary (1.degree.) and secondary (2.degree.) form. The two forms of Xenorhabdus can be differentiated most easily by their colonial morphology. Antibiotic activity is exhibited by primary form bacteria, but not by secondary forms.